1. Field of the Invention
This invention relates a copper-plating liquid, a plating method and a plating apparatus, and more particularly to a copper-plating liquid, a plating method and a plating apparatus useful for forming copper interconnects by plating a semiconductor substrate with copper to fill copper in fine recesses for interconnects formed in the surface of the substrate.
2. Description of the Related Art
In recent years, instead of using aluminum or aluminum alloys as a material for forming interconnection circuits on a semiconductor substrate, there is an eminent movement towards using copper (Cu) which has a low electric resistance and high electromigration resistance. Copper interconnects are generally formed by embedding copper into fine recesses formed in the surface of a substrate. There are known various techniques for producing such copper interconnects, including CVD, sputtering, and plating. According to any such technique, a copper is deposited on the substantially entire surface of a substrate, followed by removal of unnecessary copper by chemical mechanical polishing (CMP).
FIGS. 39A through 39C illustrate, in a sequence of process steps, an example of producing such a substrate W having copper interconnects. As shown in FIG. 39A, an oxide film 2 of SiO2 is deposited on a conductive layer 1a formed on a semiconductor base 1 on which semiconductor devices are formed. A contact hole 3 and a trench 4 for interconnects are formed in the oxide film 2 by the lithography/etching technique. Thereafter, a barrier layer 5 of TaN or the like is formed on the entire surface, and a seed layer 7 as an electric supply layer for electroplating is formed on the barrier layer 5.
Then, as shown in FIG. 39B, copper plating is performed onto the surface of the substrate W to fill the contact hole 3 and the trench 4 with copper and, at the same time, deposit a copper film 6 on the oxide film 2. Thereafter, the copper film 6 on the oxide film 2 is removed by chemical mechanical polishing (CMP) so as to make the surface of the copper film 6 filled in the contact hole 3 and the trench 4 for interconnects and the surface of the oxide film 2 lie substantially on the same plane. An interconnect composed of the copper film 6, as shown in FIG. 39C is thus formed.
The seed layer 7 is generally formed by means of sputtering or CVD. In the case where the copper film 6 is formed by electroplating with copper, a copper sulfate plating liquid, which contains copper sulfate and sulfuric acid, has generally been used as a plating liquid.
With the recent trend towards finer interconnects, the trenches for interconnects or plugs are becoming to have a higher aspect ratio. This poses the problem that a seed layer cannot be sufficiently formed by, e.g. sputtering, in the bottom portion of the trench, thus failing to form a uniform seed layer Thus, as shown in FIG. 40A, there is a likelihood that the thickness t1 of the seed layer 7 formed on the side wall of the trench near the bottom portion thereof becomes {fraction (1/10)} or less of the thickness t2 of the seed layer 7 formed on the side wall of the trench near the surface of the substrate. When electroplating with copper is carried out to fill with copper into such a trench by using a copper sulfate plating liquid, an electric current hardly passes through the extremely thin portion in the seed layer 7, causing to the formation of an undeposited portion (void) 8 shown in FIG. 40B. An attempt to overcome this drawback by increasing the overall thickness of the seed layer 7 so as to thicken the extremely thin portion would not be successful, since in electroplating with copper for filling such trench, copper would deposit thick around the opening of the trench to close it, resulting in the formation of a void.
On the other hand, a copper-plating liquid, which comprises a base such as copper sulfate and, as additives, a complexing agent and a pH adjusting agent for maintaining the liquid pH within a neutral range, has been developed. Such a copper-plating liquid, however, is generally too unstable for practical use. Moreover, the pH adjusting agent generally contains an alkali metal such as sodium and potassium. A plating liquid containing an alkali metal, when applied to a semiconductor substrate, causes electromigration to deteriorate the semiconductor. There is also known a copper-plating liquid comprising a copper cyanide. However, since cyanides are harmful to human health, it is required to avoid using such a plating liquid from operational and environmental viewpoints.
The present invention has been made in view of the above drawbacks in the prior art. It is therefore an object of the present invention to provide a copper-plating liquid which is free from alkali metals and cyanides, and which can reinforce the thin portion of a seed layer and ensures complete filling with copper in fine recesses having a high aspect ratio formed in the surface of a substrate, and also to provide a plating method and a plating apparatus which utilize the copper-plating liquid.
Tn order to achieve the above object, the present invention provides a copper-plating liquid free from an alkali metal and a cyanide, comprising divalent copper ions and a complexing agent. The inclusion of a completing agent in the copper-plating liquid can enhance the polarization as a plating bath and improve the uniform electrodeposition property. This enables reinforcement of the thin portion of a seed layer and uniform filling with copper into the depths of fine recesses, such as trenches and holes, having a high aspect ratio. Further, the deposited plating is dense, and is freed from microvoids formation therein. Furthermore, the copper-plating liquid of the present invention, which does not contain any alkali metal nor cyanide, does not cause deterioration of a semiconductor which would otherwise be caused by electromigration due to the presence of an alkali metal and, in addition, does meet the demand for avoiding the use of a cyanide.
Preferably, the plating liquid further contain a pH adjusting agent selected from agents not containing an alkali metal nor a cyanide, such as sulfuric acid, hydrochloric acid, phosphoric acid, choline, ammonia and tetramethyl ammonium hydroxide. By using such a pH adjusting agent according to necessity, the plating liquid may be maintained within a pH range of 7-14, preferably at a pH range of about 8-11, more preferably at a pH range of 8-9.
The concentration of divalent copper ions in the plating liquid should preferably be in the range of 0.1-100 g/l, more preferably in the rage of 1-10 g/l. A copper ion concentration below the above range lowers the current efficiency, thereby lowering the precipitation efficiency of copper. A copper ion concentration exceeding the above range worsens the electrodeposition property of the liquid. The concentration of the complexing agent should preferably be in the range of 0.1-500 g/l, more preferably in the range of 0.1-200 g/l, furthermore preferably in the range of 20-200 g/l. When the concentration is lower than the above range, an adequate complexing with copper can hardly be made whereby sediments are likely to produce. When the concentration is higher than the above range, on the other hand, the plating can take on the so-called xe2x80x9cburnt depositxe2x80x9d state and thus the appearance is worsened and, in addition, the treatment of waste liquid becomes different. Further, when the pH of the plating liquid is too low, the complexing agent cannot effectively combine with copper, thus failing to provide a complete complex. On the other hand, too high a pH of the plating liquid can bring about the formation of a variant form of complex which makes a sediment. The above described preferred pH range can obviate these drawbacks.
The plating liquid may also contain at least one additive selected from organic acids, amides, glycerin, gelatin, heavy metal ions, thiazoles, triazoles, thiadiazoles, imidazoles, pyrimidines, sulfonic acids, and gultamic acids.
Specific examples of the complexing agent may include ethylenediamine tetracetic acid, ethylenediamine, N, Nxe2x80x2, Nxe2x80x3, Nxe2x80x2xe2x80x3-ethylene-di-nitro-tetrapropane-2-ol, pyrophosphoric acid, iminodiacetic acid, diethylenetriamine pentacetic acid, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, diamino butane, hydroxyethyl ethylenediamine, ethylediamine tetrapropionic acid, ethylenediamine tetramethylene phosphonic acid, diethylenetriamine tetramethylene phosphonic acid, diethylenetriamine pentamethylene phosphonic acid, and their derivatives.
The present invention provides a method for plating a substrate having fine recesses, in a surface of the substrate thereof, covered with a barrier layer and/or a seed layer to fill the fine recesses with a metal, comprising: plating the surface of the substrate in a first-stage by contacting the substrate in a first plating liquid; and plating the surface of the substrate in a second-stage by contacting the substrate in a second plating liquid, wherein the first plating liquid has a higher polarization than the second plating liquid.
According to this method, when there is a thin portion in the seed layer, the thin portion can be reinforced by the first-stage plating to provide a complete seed layer, and the complete seed layer effectively serves as an electric supply layer in the second-stage plating. The method can thus fill a metal such as copper fully with the fine recesses and form a plated film having a flat surface.
The present invention, in another aspect thereof, provides a method for plating a substrate having fine recesses, in a surface of the substrate thereof, covered with a barrier layer and/or a seed layer to fill the fine recesses with a metal, comprising: plating the surface of the substrate by contacting the substrate in a plating liquid having an excellent uniform electrodeposition property.
The present invention also provide a plating apparatus comprising: a first plating section for plating a surface of a substrate having fine recesses formed in the surface thereof and covered with a barrier layer and/or a seed layer in a first-stage; a first plating liquid feed section for feeding a first liquid into a plating chamber in the first plating section; a second plating section for plating the surface of the substrate which has undergone the first-stage plating in a second-stage; a second plating liquid feed section for feeding a second plating liquid into a plating chamber in the second plating section; and a transport section for transporting the substrate from the first plating section to the second plating section, wherein the first plating liquid has a higher polarization than the second plating liquid.
The present invention provide a plating apparatus comprising: a loading/unloading section for loading and unloading a semiconductor substrate; a first metal plating unit for forming a first plated metal film on a surface of the semiconductor substrate; a second metal plating unit for forming a second plated metal film on the first plated metal film; a bevel-etching unit for etching away a metal film formed on the edge portion of the semiconductor substrate which has the second plated metal film on the surface thereof; an annealing unit for annealing the semiconductor substrate; and a transporting device for transporting the semiconductor substrate, wherein the first metal plating liquid for forming the first plated metal film has a higher polarization than the second metal plating liquid for forming the second plated metal film.
The present invention provide a plating method, comprising: forming a first plated metal film on a surface of a semiconductor substrate; forming a second plated metal film on the first plated metal film; etching away a metal film formed on the edge portion of the semiconductor substrate which has the second plated metal film on the surface thereof; and annealing the bevel-etched semiconductor substrate, wherein the first metal plating liquid for forming the first plated metal film has a higher polarization than the second metal plating liquid for forming the second plated metal film.